Exemplary embodiments of the present invention relate to detecting the conditions of tires for a vehicle, and more particularly, to an apparatus and method for detecting the conditions of tires mounted on a vehicle, which enable a driver to monitor the conditions of tires while driving by measuring rotation speeds of the tires using a plurality of wheel speed sensors and determining the conditions of the tires based on the measured rotation speeds.
In general, an apparatus and method for detecting a reduction of air pressure within a tire mounted on a vehicle may be divided into two categories. One of the categories is a direct Tire Pressure Monitoring System (TPMS), and the other thereof is an indirect TPMS.
The direct TPMS is advantageous in that it can measure accurate pressure using a method of mounting sensors on tires and performing measurement, but is disadvantageous in that the direct TPMS is more expensive than the indirect TPMS because it includes several elements, such as a pressure measurement sensor unit mounted on a tire and a radio unit configured to send measurement values in a wireless way, and that the direct TPMS has a high failure rate. The direct TPMS may include, for example, U.S. Pat. No. 4,695,823.
The indirect TPMS has slightly lower accuracy than the direct TPMS, but has price competitiveness. The indirect TPMS adopts a method of estimating a loss of air pressure using a wheel speed sensor mounted on a vehicle and configured to measure rotation speed.
The vibration of a tire can be extracted based on the rotation of the tire. A corresponding vibration frequency is associated with pressure within the tire and is distributed near about 40 Hz. The amplitude of the vibration is influenced by a load that is added to the tire. A smaller amplitude that belongs to a signal generated by the vibration of a tire can be monitored in a rear tire rather than in a front tire. For example, in the case of a rear tire, a signal variation that is caused by a mechanical error of sawteeth attached to a wheel speed sensor has a repetitive pattern, and the signal variation has some influence on a wheel speed sensor signal. In order to compensate for such a phenomenon, a sawtooth error pattern needs to be confirmed and the wheel speed sensor signal needs to be then corrected.
Since such a sawtooth error pattern has no connection with a rapid change of time, a compensation value for compensating for a mechanical error of the wheel speed sensor can be rapidly determined through synchronization between a memorized pattern that is updated while driving and an actually monitored wheel speed sensor signal using a simple exponential smoothing method. Accordingly, the present invention further facilitates the analysis and characterization of tire conditions using a tire vibration signal.
A tire resonant frequency can be estimated using a wheel speed sensor. However, the analysis of the tire resonant frequency requires a fixed time value at which a value of the wheel speed sensor is placed in an event domain, and the event is defined as the time that has elapsed between two consecutive sawteeth. For transform purposes from the event domain to the time domain, simple linear interpolation may be performed on rotation speed of the wheel speed sensor at the fixed time. If transform from the event domain to the time domain is performed, a band-pass filter may be used to separate tire torsion vibration components between 30 Hz and 50 Hz. A plurality of methods for identifying a resonant frequency is present. Fourier transform, such as that disclosed in U.S. Pat. No. 6,092,028, is one of representative examples. However, Fourier transform requires a large amount of data and also requires a heavy computational load in order to obtain desired results. As another method, a secondary linear model, such as that disclosed in U.S. Pat. No. 7,639,157, may be used. In the present invention, however, unlike in the conventional methods, a zero-crossing estimator method for detecting and counting stationary points is used.